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Production and Quality

Production and Quality

What is MTF?

       Modulation Transfer Function (MTF) of a lens is an important indicator of its performance. Both lens designers and users need to have a deep understanding of MTF.1. What is MTF?       The function of the lens is to image the object on the sensor. An object is composed of countless points. The image formed by every single point of the object will be positioned on the corresponding image point. Meanwhile, it will be weighted according to the brightness of the original target at these positions, leading to a continuous image function called "g". Unfortunately, this long formula is quite obscure.        What does the formula really stand for? In other words, what kind of phenomenon does it describe? Following is illustration by several intuitive examples.        The stripes of the pattern above (representing objects) are black and white (high contrast). From left to right, the spatial frequency of the stripes gradually increases (the stripes become denser).       The object passes through the lens and is imaged on the sensor. We can find that the large stripes in low resolution are still black and white after imaging. They are easily transferred by the lens to the sensor. As the frequency increases, the stripes become denser and the image becomes grayer, and the contrast between them becomes increasingly smaller. It no longer has perfect black and white, and even black and white are indistinguishable.        This phenomenon can be described mathematically on the "luminance distribution". The black is 1 and white is 0. At medium contrast, the brightness decreases with the grayscale of the image. The brightness distribution gets smaller and smaller until you cannot distinguish the difference between bright and dark. The right is a typical MTF curve. All MTF curves are of similar shape like this. They all have a cut-off frequency (although the cut-off frequency may be different for each lens. The cutoff frequency is related to the aperture of the lens). Then, MTF will tell you how well the lens replicates the image.2. How to interpret MTF?        Lens imaging is affected by the orientation of an object in space. For example, the pattern stripes mentioned can be horizontal or vertical. That is the Tangential and Sagittal in optical terminology. The MTF curve describes the imaging capability of the lens in both the Tangential (T) and Sagittal (S). It helps optical engineers avoid designing lenses that perform well in one direction but poorly in another.        The figure above is a classic MTF graph. The abscissa is spatial frequency in cycles per millimeter and the ordinate is modulus of the OTF. MTF is a function of frequency, so as the frequency increases, the MTF value becomes smaller.       In the top left of the figure, "TS0.00 DEG", T stands for Tangential and S stands for Sagittal. The MTF curve with a FOV of 0° is a blue line, which is what we call "on the axis". There is no difference between Tangential or Sagittal.       Next, "TS 10.00 DEG" is the MTF curve representing a FOV of 10° (green curve). The MTF in this FOV differs greatly in the T and S directions (@60lp, T0.2, S0.68). It means that the lens has relatively severe astigmatism. That is, in one focal point, imaging performs obviously better in one direction than in the other. When the ratio of them is greater than 2:1, we need to consider the effect of astigmatism on the system. Look at the MTF of the 14° field of view, the difference between the two directions of T and S is not so obvious.       Although the maximum FOV shown in the graph is only 14°, in fact, the maximum field of view of this lens is 28°. Generally, the lens is recognized to be rotationally symmetric.       To sum up, the classic MTF curve shows us how well a lens performs at a specific frequency (the number of line pairs contained per millimeter). The higher the MTF value, the better the lens can replicate the object; the lower the MTF value, the worse the lens's ability to replicate the object.3. How to judge the availability of a lens?       It is essential for optical engineers or lens users to judge whether the lens matches the sensor.        Typically, we choose lenses with an MTF greater than 0.3 at Nyquist frequencies. It is a reliable experience that lenses will not become a limitation of the whole system. Thus, do not be distracted by MTF above the Nyquist frequency, which is not the point of lens design or use. If you are wondering, is the MTF at Nyquist 0.3 or higher? When you ask this question, you already have a lens to choose from that will not affect system performance - of course, the higher the MTF, the better performance.4. Kindly reminder       At different apertures and working distances, the same lens has different optical performance (MTF). Thus, the first thing to figure out is the conditions of use regarding imaging needs before evaluating a lens.

What are the differences between dual-field lenses and continuous zoom lenses?

Both are variable lenses with variable focal length and adjustable sharpness. But there is several fundamental differences:A. Structurally, a continuous zoom lens has two sets of adjustable lenses, which are used to adjust the focus and sharpness respectively. The dual-field lens has only one set of adjustable lenses, which is used to switch focal length and adjusts the sharpness of the picture.B. In General, continuous zoom lenses require two sets of motors to control. Dual-field lenses require only one set of motors to switch the focal length and adjust sharpness.C. The focal length of continuous zoom lenses can be any value within the design range. Whereas the focal length of dual-field lenses can only be switched between the two fixed focal length.

The principle of optical passive athermalization in infrared zoom system

Optomechanical systems are typically made of several materials with different thermal properties. These materials compose the optics (refractive or reflective elements) and the mechanics (optical mounts and system housing). As the temperature of these materials change, the volume and index of refraction will change as well, increasing strain and aberration content (primarily defocus). Compensating for optical variations over a temperature range by combining, distributing focal power and selecting proper materials with reference to the differences between thermal properties of infrared optical materials is called passive athermalization.The compensation technology of passive athermalization can make the system have better temperature stability and more compact structure without move any parts or use all kinds of complex electronic equipment.The compensation design of passive athermalization mainly includes three processes:1) Select appropriate optical materials according to the temperature index, thermal expansion coefficient and other parameters of infrared optical materials. The aim is to better correct the defocus of image plane caused by temperature change.2) Select the proper combination. So that the material combination can meet the requirements of the system’s focal power while athermalization.3) Under the premise that athermalization and focal power are satisfied, the design need to be further optimized to correct the aberration of the optical system.

What are the basic characteristics of the continuous zoom structure?

(1)The focal length can be changed uniformly as well as continuously. With the increasing improvement of processing and assembling technology, the application of the zoom structure is becoming more and more common, and its function is becoming more and more convenient. In the continuous zoom system, the focal length can be uniformly changed between the maximum and the minimum focal length, and can also take any focal length value within the range.(2)The image plane remains stable during continuous zooming. The so-called image plane remains stable, usually in two aspects: first, the stability of the position of the phase plane during zooming; second, during zooming, the size of the image formed by the system remains unchanged.(3)The image quality meets the requirements. The requirement of the continuous zoom system on the imaging quality is not only good imaging quality for some unique focal length positions, but also requires the imaging quality to be as consistent as possible in the entire zoom range. Thus, it requires that the motion of the zoom group and the compensation group be well connected. The motion curve should be smooth, and unnecessary inflection points should be avoided as much as possible.

What are the contact forms when using the pressing ring and spacer to fix the lens?

According to the summary of scholars and research institutions at home and abroad, there are five main contact forms among the pressing ring, the spacer and the concave-convex lens: sharp corner, tangential, toroidal, spherical, and oblique plane.1) Sharp corner interfaceOn a pressing ring or spacer, a curved surface with a radius of about 0.05mm is ground at the intersection of the end face and the cylindrical surface. The ground surface is called the sharp corner interface, and its contact form is as follows: 2) Tangential interfaceThe interface formed by the contact between the surface of the convex lens and the surface of the pressing ring or the spacer is a tangential interface. Different from the sharp corners, the tangential interface is not used to fix the concave spherical lens, but the convex spherical lens. 3) Toroidal interfaceThe following figure shows the contact form of the toroidal interface.4) Spherical interfaceThe spherical interface contact form is that the metal pressing ring or spacer is in seamless contact with the spherical surface of the lens. Because the axial load is evenly distributed on the spherical contact surface, there is no pressure concentration in principle. However, the assembly precision of this contact form is particularly high, and the lens and the metal pressing ring must be machined with high precision, so it is difficult for average companies to accept it due to the high cost.5) Oblique plane interfaceAs shown in the figure below, the oblique plane interface is the same as the spherical interface with a comparatively big contact area, so that the contact pressure generated by thrust load is distributed evenly. But its contact surface is not real plane. The metal pressing ring or spacer surface is also not parallel to the lens surface. This kind of contact form will inevitably lead to point contact and line contact, which will cause a little bit high local pressure.

What are the commonly used cam curve design methods for continuous zoom lenses?

(1) Equal gap design. Equal gap refers to the design of the motion curve of the zoom group as a straight line. That is, the movement of the zoom group along the optical axis is linearly related to the cam angle, and the movement of the compensation group and the cam angle are non-linear. The advantages: convenient processing, high processing accuracy. The disadvantages: In some large zoom ratio systems, the lead angle of the compensation group telephoto curve will increase. It will cause that the compensation group can not move smoothly even gets stuck during the movement. Mechanically speaking, the acceleration of the variable group is infinite at the starting and ending positions during constant velocity motion, so there is a rigid impact.(2) Equiangular distance design. Equiangular distance refers the relationship between the focal length of the system and the cam angle is designed to be linear. The advantages: The focal length positioning is easy to control. It is mainly used in systems with focal length measurement accuracy. The disadvantages: During the short focal length, the lead angle of motion curve in the compensation group will increase.

Why is athermalized design needed for the infrared optical system?

Temperature variation will result in changes in all parameters of the infrared system, which will influence its image plane position and image quality. Therefore, athermalized design is required. Generally, temperature affects the infrared system in the following three ways:1.     The refractive indexes of the infrared optical components will change when the temperature changes.Under normal circumstances, the refractive indexes of the infrared optical components will change when temperature changes, which will change the focal length of the lens or the optical system.  The temperature coefficients of infrared optical materials are much larger than those of ordinary optical glass. For instance, the temperature coefficient value of K9 glass is only 2.8x10-6C-1, while that of the single crystal germanium dn/dt (a commonly used material for making infrared lenses) is 396x10-6C-1, about 141 times larger than the former. Therefore, the influence of temperature on the refractive index is quite apparent in the infrared system.2.     The radius of curvature and the center thickness of the infrared optical components will change when the temperature changes.This change is caused by the fact that the material of the components expands on heating and contract on cooling, which is related to the optical material’s linear thermal expansion coefficient (a0). When the temperature changes, its radius of curvature and center thickness will turn into:D' =D+dD=D+D* a0*dTR' =R+dR=R+R* a0*dTNote: R and R' are respectively its radius of curvature before and after temperature change; D and D' are respectively its center thickness before and after the temperature change. dT refers to the temperature variation.3.     The thermal effect of the lens tube materialWhen the temperature changes, the dimensions of the assembly material will change, which will cause a change in the air gap between the optical components. Ultimately, it will influence the image quality. This change is associated with the assembly material’s linear expansion coefficient.Among the above three main factors, the change of the refractive index of the optical material has the greatest influence on the image plane position and image quality; the influence of the radius of curvature is the second greatest, while changes in the thickness of optical components and the space between them have the least influence.

How do the distance and angle affect the thermal imaging temperature measurement?

Without considering atmospheric transmittance and radiative transmission loss,A.    To the source point, the irradiance of the object at the source point is inversely proportional to the square of the distance. Although the radiation intensity of the source point does not change, the field angle between the source point and the thermal image becomes smaller as the distance between them increases. The figure below shows the relationship between the irradiance of the source point and the distance. Thus, when the distance increases, the measuring temperature will get obviously lower.A.    To the plane source object, if there is no angle between the target light source and the thermal imager at the same plane, and the instantaneous field of view in the measurement system is full of radiation energy, the distance change will make no difference in the result of thermal imaging temperature measurement. 

What are the main factors that can affect thermal imaging temperature measurement?

Many factors can affect thermal imaging temperature measurements, such as the emissivity of the target, the radiation temperature of the target, the atmospheric radiation temperature, the ambient reflection temperature, and other external factors; meanwhile, it is also affected by the lens temperature, the temperature of the detector array, the temperature of the inner thermal camera module, the temperature of the system circuit, and other internal factors. Moreover, in the actual temperature measurement process, the distance of the object and the angle also influence the measurement result.

What are the evaluation indices of Infrared Machine Core Focusing Functions?

1.Sensitivity. Near the peak of the focusing function, when the abscissa changes simultaneously, the y-coordinates of different focusing curves change to various degrees. The more significant is the y-coordinate variation near the peak value, the easier it is to find the real focal plane.2.Width of the Steep Region of the Curve. In the focusing process of the infrared lens, as it deviates further and further from the peak value, the image at first becomes increasingly blurred, and the function value decreases sharply until almost nothing can be seen distinctly; then, the curve becomes much flatter and the function value shows no significant change. We divide the focusing curve into steep and flat regions to represent this feature.In the steep region, the focusing function value changes sharply with the change of focusing distance indicated by the abscissa, while in the flat region, the function value virtually shows no change.3.Degree of Steepness. The first defocus stage and the second one of the infrared lens are different, because the front and the rear of the infrared detector have different depths of field and receive various amounts of radiation. Hence, the focusing curve does not show the same degree of steepness at the two sides of the peak value.4.Fluctuation Quantity of the Flat Region. The pattern of the focusing curve is influenced by the external radiated noise. Therefore, in a non-ideal state, the function value of the focusing curve will see some noticeable fluctuation in the flat region, and the quantity of such fluctuation is used to describe the degree of the fluctuation. This quantity can reflect the anti-noise ability of the focusing curve; the smaller the fluctuation volume (V) is, the more stable is the anti-noise ability of the function.5.Time. The computing time (T) represents the focusing speed of a function. The result of the calculation of each function is achieved by the test under specific conditions. Therefore, when the test conditions change, the result will change accordingly.

What are the current technical proposals for FOV stitching?

A. image space  stitching (internal  stitching) refers to the image stitching using multiple detectors, which can be done directly, or by using optical elements. In both ways, inner stitching arranges the detectors in a certain order to form a whole receiving target plane. Some have proposed stitching by reflective prismatic decomposition, dividing the image plane into four parts which respectively belong to four detectors; it requires no moving components and thus maintains a compact structure. However, it necessitates a relatively long back focal length (BFL) to allow for the placement of the reflecting prism. Moreover, the energy efficiency would be low, and the lateral dimension of the system would be large. Within the infrared band, the focal planes of cooled detectors are enclosed in the Dewar flask, therefore, the direct stitching of the focal planes is inappropriate, and considering the lengths of the detectors, stitching by reflective prismatic decomposition or time-share pointing would require a larger BFL thus a longer overall structure.B. Object stitching (Outer stitching) is the stitching of the observation area in the object space. It involves the placement of multiple cameras in certain relative angles and positions to obtain an image of a large field of view. Each camera lens is responsive to a detector, for instance, “horizontal 1”, “vertical 1”, crisscross pattern, and “T” type. Some theses have put forward the crisscross pattern stitching of four surveying cameras to achieve the measurement of a large field of view. This is a simple arrangement, yet it requires cameras with the same optical parameters, and the optical parallax is inevitable, which brings about inaccuracy of FOV stitching. The cooled infrared detector is quite expensive, thus increasing the processing cost. 

What are the current mid-wave and long-wave dual-band infrared imaging technologies?

1. Dual-detector and dual-band imaging technologyThe method uses two detectors with different wavelength bands and separate optical systems to construct dual-band imaging, and then obtains dual-band images through image registration and fusion technology.2. Dual-line and dual-band imaging technologyTwo-band detection line arrays are placed side by side on the same focal plane to obtain a line array detector that can detect dual-band radiation. At present, only the US Naval Research Laboratory has reported the research results of the dual-column dual-band imaging system in the open literature.3. Single-detector and dual-band imaging technologyThe development of dual-band detectors transferred from quantum wells to mercury cadmium telluride materials has accelerated and has begun to be commercialized.

How are infrared optical lenses designed and produced?

1. Determine the use band according to the usage scenario, system requirements, cost requirements, etc. Most of the occasions use a single band such as short wave, medium wave or long wave, but some special occasions need to use multiple bands. 2. After the band is determined, according to the overall performance requirements of the infrared system, the optical path layout is carried out, the technical parameters such as the focal length and the field of view of the infrared lens are determined, and the imaging quality requirements of the lens are determined at the same time.3. Select the initial structure form of the lens according to the performance index and optical path layout of the infrared optical lens. The initial structure is a typical structure form summed up in the case of a lot of design experience. The following is a brief list of a few: 4. Perform aberration correction and other optimizations on optical lenses through optical design software or other auxiliary tools. 5. Image quality evaluation of corrected and optimized optical solutions. 6. Calculate, assign and formulate processing tolerances and assembly tolerances of infrared optics components and assemblies.7. Draw infrared optical system diagrams, components and parts diagrams, etc. 8. All drawings are transferred to trial production drawings and start production trial production. 9. Test/debug the trial samples, issue test reports, optimize defects,10. Officially put into mass production and use.

How does the long EFL with large optics capabilities affect the DRI?

The focal length determines the field of view (FOV) of the thermal imaging camera. The longer the focal length, the smaller the FOV, which translates into more pixels across a target at a fixed range (meaning, the target angle divided by the IFOV angle).